A multiplicity of stars

May 9, 2006The number of planets detected outside our solar system increases almost weekly. Astronomers have found 188 extrasolar planets  planets orbiting stars other than the Sun. Our own single-star system appears to be less common than multiple-star systems. Chandra X-ray Observatory observations indicate more than 80 percent of all stars reside in multiple-star systems. Scientists who study habitable planet formation in multiple-star systems suggest about 25 percent of known giant extrasolar planets are members of multiple-star systems.

Deepak Raghavan of Georgia State University and his colleagues studied 131 exoplanet systems  star systems, other than our own, that have orbiting planets  to ascertain whether they contain single or multiple stars. The team announced in March its finding that 30 (23 percent) of the extrasolar planetary systems studied contain more than one sun. Although the number is lower than the Chandra observations indicate, the study included a majority of giant extrasolar planets. The scientists' finding is consistent with expectations and indicates multiple-star planetary systems are likely places to find planets, giant or otherwise.

The scientists studied extrasolar planetary systems detected using the radial-velocity method. Scientists use the radial-velocity technique to measure changes in a nearby star's radial velocity  the line-of-sight velocity component of a star as it moves toward or away from Earth. Changes in the star's radial velocity are due to the gravitational effects an orbiting body has on the star. The radial-velocity technique is the most successful exoplanet-detection method to date. Scientists have found 176 exoplanets using this method since finding the first in 1989.

Although successful, this method is limited to stars up to about 160 light-years from Earth and biased toward finding giant, Jupiter-size planets. Giant planets' radial velocities are easier to detect than smaller, less-massive planets.

The scientists excluded other detection methods, summarized below, because the extrasolar planetary systems discovered are too distant to study their host stars adequately.

Gravitational microlensing: Scientists observe a distant background star's light being bent and magnified as a planet and its host star pass between it and Earth. Microlensing events are rare, and this detection method has yielded only 4 exoplanets to date. However, future space-based observations of gravitational microlensing events may prove successful at detecting more terrestrial exoplanets  small, rocky, earthlike planets.

Pulsar timing: Scientists detect anomalies in a pulsar's pulsation period, which is more precise than an atomic clock. Scientists discovered 4 pulsar planets  planets orbiting a dead neutron star called a pulsar  orbiting PSR 1257+12. Three of the exoplanets (PSR 12657+12b, c, and d) were discovered in 1992 and one (PSR B1620-26b) in 1994. Two of the exoplanets are about Earth-size, but intense radiation from the pulsar would almost certainly leave all four exoplanets unsuitable for life to develop.

Transit: Scientists observe changes in a star's brightness as an object, presumably a planet, crosses in front of it. Few stars are oriented such that astronomers can use the transit method. However, the transit method can be used on distant stars to detect small terrestrial exoplanets. Scientists expect to find more earthlike exoplanets with future missions like NASA's Space Interferometry Mission and Kepler Space Mission, the European Space Agency's Darwin, and France's COROT.

The scientists' results will appear in a future issue of the Astrophysical Journal.